1. Field of the Invention
Urea granules or prills which have been coated with sulfur to slow the rate of dissolution of the urea in the soil when they are applied as a fertilizer can be coated with a sealant to further slow the release of urea. This invention relates to novel sealant compositions which are comprised of mixtures of fatty esters of polyhydric alcohols and polymers which are soluble therein and contain ester, carboxylate, or hydroxyl functionality; said mixtures are preferably crosslinked with a transesterification catalyst, and optionally contain dicarboxylic or tricarboxylic acids with 9 to 60 carbon atoms, or mixtures thereof.
2. Description of the Prior Art
The process for sulfur coating of urea granules or prills was developed in 1968 by the Tennessee Valley Authority (TVA) in Muscle Shoals, Ala., as an economical system for reducing the rate of dissolution of urea particles when they are applied to the soil as fertilizer. U.S. Pat. No. 3,342,577 describes this sulfur coating process and also the sealant material that was necessary to fill in the holes that result naturally in the sulfur coating as it cools. The TVA process is also described in Sulphur Inst. J. 4(3) 2-6 (1968), ibid. 8(4) 2-5 (1972a) and ibid. 8(4) 6-8 (1972b). The TVA recommendation for the sealant now used extensively in the manufacture of sulfur coated urea is a mixture of 30% polyethylene resin in 70% bright stock mineral oil.
U.S. Pat. No. 4,042,366 describes a modification of this process in which particles of water soluble nutrients are embedded in a soft petroleum wax coating to provide anticaking properties to the coated particles as well as nutrient benefits.
The process is further described in Sulfur Coating of Urea Treated with Atapulgite Clay, Gullett, G. L.; Simmons, C. L.; and Lee, R. G.; presented at the 198th American Chemical Society meeting in Miami Beach, Fla., in September 1989.
The requirement for a sealant for sulfur coated urea (SCU) has been documented by McClellan and Scheib (Sulphur Inst. J. 9(3/4) 8-12 (1973), and by Scheib and McClellan ibid. 12(1) 2-5 (1976).
A description of slow release urea and NPK fertilizers is given in Hort. Rev. 1 79-140 (1979).
The wax-oil sealants as described by TVA publications and currently being applied to SCU require the addition of a clay conditioning agent at levels nearly equal to that of the wax sealant to prevent caking and provide a free flowing product. A typical process may require 3% of the wax-oil sealant and 2% of the clay conditioning agent on the weight of the SCU, or 67% clay on the weight of the sealant. This requires large scale clay handling equipment in addition to the equipment necessary to apply the molten sulfur and wax sealant.
When the clay conditioned SCU is applied in the field by mechanical spreaders, the clay or wax-clay mixtures tend to be removed from the SCU particles by the abrasive action of the screw conveyors and the mechanical spreading wheel, resulting in a build-up of wax-clay on various parts of the machine and requiring frequent shut-downs for cleaning.
An additional problem with SCU from current production methods is the reduction or loss in water insoluble nitrogen (WIN) which occurs on shipping the SCU from the production point to the local blending point. This loss is exacerbated by the further abrasion which occurs in the blending and bagging operations. Urea and blended fertilizers when they are mixed and bagged must be labelled with their WIN content, and it is the responsibility of the manufacturer to assure that the value does not decrease below the labeled value during shipping and storage. Thus a significant loss in WIN resulting from handling in modern high-speed equipment can create a situation in which the fertilizer is mislabeled and subject to recall.
Hydrogenated tallow and related esters have been used to obtain sustained release of pharmaceutical materials. U.S. Pat. No. 4,132,753 describes a process for mixing such materials with finely divided glyceride wax type materials with radiant heat to imbed the pharmaceutical materials in the wax materials. Eur. Pat. Appl. EP 383,406 describes preparing a dispersion of an active ingredient in molten fatty acid derivatives and spraying into a cooled gas stream to form the encapsulated active materials.
U.S. Pat. No. 4,401,782 describes mixtures of hydrogenated tallow and acrylic acid-ethylene copolymers, and U.S. Pat. No. 4,253,840 describes mixtures of hydrogenated tallow and ethylene-vinyl acetate copolymers, both of which are used to prepare textile sizing materials.